If you're uploading more than 1000 items into an auction, it's recommended that you split the file into multiple parts or create multiple auctions (e.g. "Day 1", "Day 2") for performance and stability reasons.
You can either do this manually in Excel or use an external tool to help divide the file into smaller sections. There are many open source software tools available that can assist with this task, but always download responsibly.
Make sure each file keeps the first two rows from the original template as these rows are required for a successful upload.